The microbiome is the ecological community of commensal, symbiotic, and pathogenic microorganisms that share our body space (Medical and Health Genomics, 2016, page 15–28). The human gut is the location where the maximum number of microorganisms can be found. Among the different microorganisms they can be broadly classified into two groups: the beneficial and harmful. In the human gut there is always a balance between the beneficial and the opportunistic microorganism which maintains human health. However, if the balance is not maintained and homeostasis is disturbed, with an increase in opportunistic microorganisms, it may result in various diseases like inflammatory bowel disease, irritable bowel disease, ulcerative colitis, Crohn's disease, colorectal cancer, metabolic disorders and neurodegenerative diseases including motor neuron diseases. In the present chapter we discuss the role of gut bacteria in motor neuron diseases like multiple sclerosis, Parkinson's disease and amyotrophic lateral sclerosis.

微生物组是共享我们身体空间的共生、共生和病原微生物的生态群落（Medical and Health Genomics，2016，第 15-28 页）。人体肠道是微生物数量最多的地方。在不同的微生物中，它们可以大致分为两类：有益的和有害的。在人类肠道中，有益微生物和机会微生物之间始终保持平衡，从而维持人类健康。但是，如果不保持平衡，扰乱体内平衡，随着机会性微生物的增加，可能会导致各种疾病，如炎症性肠病、肠易激病、溃疡性结肠炎、克罗恩病、结直肠癌、代谢紊乱和神经退行性疾病，包括运动神经元疾病。